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What is the best (or standard) way to get coax from the radio in the car to the outside of the car without smashing it and damaging the line?

If I just put it through the door crack, it seems to look pretty bad and I don't want to damage my new magmount.

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3 Answers 3

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Best would be to cut a hole in the car and feed the coax through that. There are antenna mounts designed to bolt to a car body panel and take care of structurally supporting the antenna and forming a weatherproof seal. NMO mounts are pretty common for VHF antennas.

If you don't want to modify your car in any way, use a section of small diameter coax to slip through the rubber seal around a door or trunk if your feedline is too fat. You can use small connectors like SMA to connect it to your feedline. Just be sure the connections are appropriately weatherproofed to avoid getting water in the feedline.

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    $\begingroup$ It's worth noting that passivated glass (used in most newer cars) dramatically increases the losses in glass-mount antennas. It's like adding an additional plate to the capacitive coupling. $\endgroup$
    – David KF4MDV
    Commented Jan 26, 2015 at 17:01
  • $\begingroup$ @DavidKF4MDV Interesting...I didn't know about that. I've removed the suggestion since it sounds like it's pretty horrible. $\endgroup$
    – Phil Frost - W8II
    Commented Jan 28, 2015 at 13:36
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The other alternative is to go through the trunk, which often has easier access than other locations on the car.You can usually get access to the trunk from most modern cars, and that would allow you to have easy access.

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Best position for an antenna is the middle of the roof, if you remember your ARRL Technician exam. You can route your cable from somewhere in the dash, to the center or B pillar. From there you can probably use the crevices to get the coax out.

You just mentioned coax, so I'm assuming you will be using standard RG-58. It's the typical coax used in many installations. I'm using it too but I have now started mixing it with RG-174. It is a smaller diameter and very flexible coax. It can fit into some areas more nicely but given it's size it can be easily destroyed or frayed (i.e. jammed between two metal objects). RG-174 can be used to replace your standard RG-58, just mind the crevices that it has to enter. Use extra care when routing. Use cable protectors liberally, you can use the ones used for the car's internal wiring or get a new one that will carry only the coax.

Another option will be to use a "through window cables". A through window cable is a very flat piece of cable with connectors at each end. It is available in 50 ohms if you require correct it. Amateur radio antenna manufacturers have this item in their catalogs. Watch your power output when using through window cables so as not to exceed the ratings.

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    $\begingroup$ I've seen some through-window cables that are twin-lead, frequently simply printed on a flex pcb. These may be 50 ohms in free space, but when shut in a window and pressed near the metal body of the car, they won't be 50 ohms anymore. $\endgroup$
    – Phil Frost - W8II
    Commented Jan 27, 2015 at 19:00
  • $\begingroup$ Have you used one? It would be a good point to measure the impedance after installation and disprove the manufacturer claims. I'm using a 75 ohm for my room and RX-only radio. $\endgroup$
    – sessyargc.jp
    Commented Feb 2, 2015 at 0:50
  • $\begingroup$ Have you seen one that claims to maintain its impedance even when installed very near a sheet of metal? For Rx-only, it probably doesn't matter much. For transmitting it's probably a bigger deal, especially at VHF where the length of the ribbon is a significant fraction of wavelength. $\endgroup$
    – Phil Frost - W8II
    Commented Feb 2, 2015 at 12:41

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